1. Field of the Invention
The present invention relates to a method and an apparatus for controlling vibration, and particularly, a method and an apparatus for controlling vibration which is capable of reducing the vibration of a substance caused by vibration reverse from a vibration source by exciting a vibration to the above transmitted vibration by means of a vibration exciter.
2. Description of the Related Art
As a vibration control method of this type, there is known a technique of reducing a vibration on the basis of a feedforward signal using an adaptive filter in which vibration information of a vibration source is taken as a reference input and vibration information of a vibration-proof object is taken as an error input.
FIG. 1 is a view showing the construction of this prior art vibration controller.
For vibration, six degrees of freedom must be taken into account; however, herein, only one directional component will be examined for simplification.
A vibration-proof object 2 such as a precision instrument is supported on a ground 1 through an equivalent supporting system 3 which equivalently supports the vibration-proof object 2 by a spring and a damper.
A vibration exciting actuator 4 is additionally provided on the vibration-proof object 2 for forcibly applying vibration to the vibration-proof object 2.
A vibration sensor 5 for detecting a vibration transmitted from a vibration source is provided on the ground 1, and a vibration sensor 6 for detecting a vibration of the vibration- proof object 2 is provided on the vibration-proof object 2.
On the other hand, a feedforward controller 10 for outputting a drive signal to the vibration exciting actuator 4 uses an adaptive filter 11, and which updates a filter coefficient by the FX algorithm of a filtered-X LMS 12.
Detection signals from the vibration sensors 5 and 6 are amplified by amplifiers 17 and 18 and are converted into digital signals by AD converters 15 and 16, respectively. The converted digital signals are inputted in the feedforward controller 10. A drive signal is then outputted from the adaptive filter 11, and is converted into an analog signal by a DA converter 19. The converted analog signal is amplified by an amplifier 20 and is inputted in the vibration exciting actuator 4, to thus drive the vibration exciting actuator 4.
The calculation necessary for the vibration control by the above adaptive filter is shown as follows: ##EQU1##
In the above expressions (1) to (4), W.sub.i (i=0, 1, 2, . . . , N1) is a filter coefficient of the adaptive filter; x(n) is a sample value of the vibration information of a vibration source as a reference input to the adaptive filter at a time n; y.sub.F (n) is an output value from the adaptive filter; P.sub.i (i=0, 1, 2, . . . , M1) is a value approximated by discretion of an impulse response of an acceleration on the vibration-proof object 2 with respect to the input of the vibration exciting actuator 4, which is previously measured; .mu. in the expression (4) is a convergence parameter and is usually selected to be a number between 0 and 1; and e(n) is a sample value of the vibration information of the vibration-proof object 2 as an error signal at a time n.
The number of the filter coefficients in the expression (2) are dependent on the length of the impulse response of the actual actuator and the vibration-proof supporting system. For example, in the vibration system with a low natural frequency such as a vibration removing base, since the impulse response is long, a large number M of the filter coefficients are required. As a result of which the calculation amount is enlarged as a whole.
The effect of the reduction in vibration is shown in FIG. 2. As is apparent from this figure, the transmitted periodic vibration component can be relatively effectively reduced; however, generally, the characteristic frequency of the vibration system is difficult to be reduced.